Prior work has shown that tumor infiltrating lymphocytes (CTL) may be expanded in vitro and, when transferred to tumor bearing mice in combination with interleukin-2 (IL-2), can reduce lung metastases and prolong host survival. Studies have been undertaken to optimize the efficacy of in vivo TIL function. 1. In vitro growth of TIL in low dose (20 u/ml) versus high dose (1000 u/mll IL-2. During in vitro expansion, TIL require IL-2 and repeated syngeneic tumor stimulation. TIL growth in vitro at low dose IL-2 was found to be at least 3-fold more potent in vivo in the treatment of 3 day lung metastases than TIL grown continuously in high dose IL-2 or TIL switched from low dose to high dose IL-2 as little as 4 days before adoptive transfer. 2.TIL potency and nitric oxide synthase inhibitors (NOSI) N-methyl-arginine (NMA), a NOSI, has been shown to augment the allo- reactivity of murine splenocytes in vitro. After confirmation of these findings in this laboratory, NMA was found to minimally augment TIL cytotoxicity in vitro. This result does not appear to be explained by significant differences in cytokine secretion by TIL in response to tumor. Although significant responses to immunotherapy of established cancers using TIL and IL-2 have been observed in animal models and in patients, further optimization of in vivo TIL function is necessary to improve the results in ongoing clinical trials.